Pulse radiolysis, radical identification

A century ago, the reduction of silver ions in photographic plates helped W. Roentgen [1], then H. Becquerel [2], to discover x-rays and radiation of radioactive elements, respectively. Various metal ions were subsequently used widely in aqueous solutions as radical scavengers and redox indicators of the short-lived primary radiolytic species, allowing their identification and the calibration of their yield of formation [3-5]. Some underwent reduction by y [6] or pulse radiolysis [7] to the zero-valence metal, to form colloids and then precipitates [7,8]. 

Similar three-electron-bonded intermediates must also be formed in the well-known thiyl radical-induced scrambling of disulfides (Owen and Ellis 1973). Mechanistic details have been studied using the pulse radiolysis technique which allowed the identification the three-bonded intermediate and the determination of some equilibrium constants, but also gave evidence that this system may be more complex than described by reactions (42) and (43) (Boni-facic and Asmus 1984). 

Wardman P, Dennis MF, Everett SA, Patel KB, Stratford MRL, Tracy M (2003) Radicals from one-electron reduction of nitro compounds, aromatic N-oxides and quinones the kinetic basis for hypoxia-selective, bioreductive drugs. Biochem Soc Symp 61 171-194 Warman JM, de Haas MP, Hummel A, van Lith D, VerberneJB, Loman H (1980) A pulse radiolysis conductivity study of frozen aqueous solutions of DNA. Int J Radiat Biol 38 459-459 Warman JM, de Haas MP, Rupprecht A (1996) DNA a molecular wire Chem Phys Lett 249 319-322 Warters RL, Lyons BW (1992) Variation in radiation-induced formation of DNA double-strand breaks as a function of chromatin structure. Radiat Res 130 309-318 Warters RL, Hofer KG, Harris CR, Smith JM (1977) Radionuclide toxicity in cultured mammalian cells Elucidation of the primary site of radiation damage. Curr Top Radiat Res Q 12 389-407 Weiland B, Huttermann J (1998) Free radicals from X-irradiated, dry and hydrated lyophilized DNA as studies by electron spin resonance spectroscopy analysis of spectral components between 77 K and room temperature. Int J Radiat Biol 74 341-358 Weinfeld M, Soderlind K-JM (1991) 32P-Postlabeling detection of radiation-induced DNA-damage identification and estimation of thymine glycols and phosphoglycolate termini. Biochemistry 30 1091-1097... 

The experiments performed to study the TBP degradation mechanism (18, 279-287) consisted mainly of the identification of final products, but also included the examination of the radical intermediates by ESR (288, 289) or by pulse radiolysis and... 

Scientists should focus on the initial steps of the processes (identification of the transient radical species with dedicated techniques like ESR or pulsed radiolysis), the kinetic aspect, and sensitive external parameters, like the nature of the radiation (a, p, y e ), to control the accumulation of radiation-induced damage. Dedicated tools need to be built. 

Dorfman and collaborators have recently developped a very promising technique for the production of carbenium ions as transient species in halocarbon sdvents based on the dissociative ionisation of suitable precursors induced by pulse radiolysis of the solvent. While the extremely interesting kinetic results vdiich this group is obtaining will be discussed in Sect. II-G4, it is emphasised here that the fast time response of the apparatus used allows the characterisation of carbenium ions hitherto unobservable because of their excessive reactivity. The ultraviolet absorption spectrum and some reactions of the benzylium ion have been studied for the first time wdth this powerful tool. From the point of view of cationic pdymerisation, the information obtained in this type of work is particularly relevant, since it deals vrith the identification and reactivity of carbenium icais formed in very low concentration in the nght kind of medium. Cation radicals had already been prepared by pulse radiolysis involving nondissociative ionization (electron ejection or transfer), as will be discussed in Sect. II-K. 

The spectrum of the transient, shown in Fig. 17, agrees with that reported for aT radical-anion obtained by pulse-radiolysis of solutions of a-methyl styrene498. This justifies the identification of the transient with ar, K+. 

Recent flash photolysis spectra for 4,4 -DPE reveal two transients in water-containing alcohols, one species with lmax = 460nm and one with 500 nm the former is assigned to a radical of the type H-DPE, the latter to its protonated form, H2-DPE + [484-486], An example of the absorption spectra of the two radicals of 4,4 -DPE is shown in Figure 19. Previous ESR [487] and recent pulse radiolysis studies [488] confirm this assignment. Formation of radicals by pulse radiolysis and laser photolysis (e.g., photoionization in polar solvents) are useful for the identification of intermediates in photoprocesses bypassing isomerization [172, 489-491]. 

Pulse radiolysis shows that the pyrimidine radical cations are fairly strong acids and rapidly deprotonate at a heteroatom [reaction (98)]. As protonation/deprotonation reactions at heteroatoms are easily reversible, the radical cations are regenerated upon reprotonation. Deprotonation at carbon or reaction with water yields the final free-radical products [reactions (99) - (101)]. It is noted that in thymidine [23] and 5 -thymidylic acid [104] the allylic thymine radical is observed by EPR and there is very little question that its precursor is the thymine radical cation. The identification of the C(6)-OH-5-yl radical by EPR supports the view [100] that reaction with water competes with the deprotonation at methyl. Due to the ready oxidation of the (reducing) C(5)-OH-6-yl by peroxodisulfate, this type of radical is only observed at low peroxodisulfate concentrations in these systems, i.e. the (oxidizing) C(6)-OH-5-yl radicals are correspondingly enriched under conditions favourable to a chain reaction [22]. In the case of 1,3-dimethyluracil the interesting characteristics of... 

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